The present invention relates generally to semiconductor devices, such as Field-Effect Transistors (FETs).
Gallium Arsenide (GaAs) Metal-Semiconductor Field-Effect Transistors (MESFETS) are well known devices for providing amplification at microwave frequencies, high-speed digital switching and various other functions in different applications. The use of microwave devices in satellite-based and wireless communications has also grown in recent years. There is large market in such areas for MESFETs having very high power capability per unit length of periphery, typically expressed in watts/mm. As the power capability or output of transistors improves, a single transistor can provide the power that in previous generations was provided by multiple transistors, which results in a cost, volume, and weight savings. Moreover, the greater the power capability of a transistor, the broader its potential applications and the larger the potential market. Thus, there has been a great deal of activity directed toward improving the performance of transistors for different applications including military, industrial and commercial applications. Operating these transistors at higher voltages is a preferred method for increasing the operational power density of a FET capable of supporting the higher operating voltages.
Some conventional high voltage MESFETs have a positive temperature coefficient as described, for example, in U.S. Pat. No. 6,559,513, which is hereby incorporated by reference herein in its entirety. A positive temperature coefficient can cause a drain current of a FET to increase when a temperature increase occurs under a fixed bias (e.g., constant applied gate and drain voltages). As a result, the FET circuit heats up even more, which then causes the drain current to further increase. This positive feedback phenomenon can result in a problem called “thermal runaway.” This “thermal runaway” behavior is often experienced in heterojunction bipolar transistors (HBTs), for which base and emitter ballasting techniques are known for compensating for the behavior.
The positive temperature coefficient inherent in high voltage FET technology also causes significant thermal asymmetry to occur in an uncompensated large array of FETs having gate connections or fingers used, for example, in output stages of high power amplifiers. These thermal asymmetries can cause premature circuit failure, poor robustness and poor power performance. Accordingly, it is desirable to address the problem of thermal runaway so that FETs having a positive temperature coefficient can be more efficiently operated and provide a longer useful life.